The present invention relates generally to the field of analysis of electroplating baths. In particular, the present invention relates to the analysis of organic additives in electroplating baths.
Electroplating baths for copper and other metals are typically aqueous, or mostly aqueous, solutions composed of metal compounds or salts, ionic electrolytes, and various additives such as brighteners, suppressors, levelers, accelerators, surfactants, defoamers, and the like. These electroplating baths, which are used to deposit metals or semimetals such as copper, nickel, gold, palladium, platinum, ruthenium, rhodium, tin, zinc, antimony, or alloys such as copper-tin (brass), copper-zinc (bronze), tin-lead, nickel-tungsten, cobalt-tungsten-phosphide, and the like are used in applications such as the fabrication of electronic devices and components, such as conductive circuits for printed circuit boards, multichip modules, semiconductor devices and the like.
Reliable operation of these electroplating baths in a manufacturing process requires that methods of analysis are employed to determine the appropriate concentrations of the reagent species for bath startup. These analytical methods are also used to determine the concentrations of species in the bath during operation, often with on-line feedback control, to allow the components of the bath to be monitored and adjusted as required to maintain concentrations within pre-determined limits. Bath analytical methods are also used to determine the chemical identity and concentrations of species that are produced in the bath as a consequence of chemical and electrochemical reactions that take place during bath operation and/or idling.
Electrochemical methods are used principally for the analysis of acid copper plating baths used for plating circuitry on printed wiring boards and integrated circuits. Besides the inorganic components of these plating solutions (copper ions, sulfuric acid and small amounts of chloride ions) the baths contain one or more organic additives (brighteners, suppressors and levelers). In the proper concentrations, these organic additives give a bright, smooth deposit with excellent mechanical and electrical properties.
Analyses of plating bath additives are described by Tench and coworkers in U.S. Pat. No. 4,132,605, and Fisher in U.S. Pat. No. 4,917,774. These methods were devised to measure the brightener concentration in a bath containing a suppressor and inorganic components only. These methods cannot measure the presence of a leveler component in a plating bath containing brightener and suppressor.
The electrochemical methods for plating bath analysis described by Tench et al. and Fisher rely on the fact that the brightener and suppressor work in opposition to one another with respect to their effect on the potential of an object being plated. Suppressors, as their name implies, increase the overpotential for plating and thus suppress the plating rate for any given electrical energy input to the bath. In the presence of suppressors, brighteners lower the plating overpotential and cause the plating rate to increase for any given input of electrical energy to the plating bath. Suppressors cause an abrupt suppression of the plating rate at very low concentrations, on the order of 50 parts per million or less. Above that threshold level the plating overpotential changes very little, if at all. Suppressor concentrations are usually kept in the range of several hundreds to several thousand parts per million to ensure that the suppressor concentration is always well above the threshold value.
U.S. Pat. No. 4,917,774 describes a stepped potential method wherein a three electrode cell is employed. The electrodes are 1) a working electrode, 2) a counter electrode, and 3) a reference electrode which are all immersed in the plating solution to be analyzed. The working electrode is a noble metal such as platinum in the form of a rotating disc. The disc is sealed at one end of a Kel F rod and is rotated during the analysis to ensure that uniform hydrodynamic conditions prevail. The potential of the working electrode is controlled by input from a potentiostat slaved to a computer. For any working potential required, the computer will direct the potentiostat to set the potential difference between the counter electrode and the reference electrode to give the desired potential at the working electrode. The computer and potentiostat can be embodied in a single unit such as the Electroposit(trademark) Bath Analyzer, (Shipley Company, Marlborough, Mass. and S-Systems, Norwood, Mass.). The potential of a working electrode is held at various potentials in a plating solution to clean, equilibrate, plate and strip the plated deposit from the electrode. For example, the cleaning step comprises holding the working electrode at from 1.6 to 2.0 V for from 5 to 30 seconds, the equilibration potential at from 0.5 to 0.6 V for 10 to 60 seconds, the plating potential at from xe2x88x920.2 to xe2x88x920.3 V for 1 to 10 seconds, and the stripping potential at 0.2 to 0.3 V for 5 to 30 seconds. A measurement of the initial current flowing during the plating step is directly related to the brightener concentration in the plating solution. When using the Electroposit(trademark) Bath Analyzer, the initial plating current is displayed as Total Brightener Analysis Units (xe2x80x9cTBAxe2x80x9d) units, hereinafter referred to as TBA analysis.
Levelers are often present in plating baths. Like suppressors, levelers cause a reduction of plating rate for any given electrical energy input to the plating bath. Unlike suppressors, whose effect is general in nature, levelers cause a localized depression in plating rate. They act under mass transfer control to suppress the plating rate by adsorbing at locally higher potential regions of the article being plated. The above described electrochemical methods cannot be used when levelers are present in the bath. This is due to the leveler""s suppressing effect on the plating rate which varies with its concentration in the bath. To properly analyze for brightener when leveler is present the leveler concentration must already be known. The techniques described above provide no solution for overcoming the confounding effect of the leveler on the brightener, and therefore cannot be used when these two additives coexist in a plating bath.
U.S. Pat. No. 5,223,118 (Sonnenberg et al.) describes an analysis method that measures both brightener and leveler coexisting with suppressor in a plating solution. In this method the brightener concentration is first determined by the TBA method and, if necessary, adjusted by external addition to a value that gives maximum sensitivity for analysis of leveler. The method then uses a freshly prepared copper electrode to monitor the energy input with time to the electrode while plating at an applied current held at a constant value. Such copper electrode is prepared by first plating copper from a separate copper electroplating solution free of organic additive on the electrode. The slope of the resulting energy-time plot is used to quantitatively determine the leveler concentration. However, this method only works with a narrow range of low concentrations of brightener and only within a specific range of leveler concentrations, the analytical procedure is lengthy (ca. 10 minute equilibration period) and is not a real-time analysis. This method measures the plating potential, the slope of which with time is dependent upon the concentration of leveler. Such method has not achieved acceptance in the semiconductor industry.
Other bath analysis methods, such as AC impedance, high pressure liquid chromatography (xe2x80x9cHPLCxe2x80x9d), ion chromatography (xe2x80x9cICxe2x80x9d), titrimetry, gravimetric analysis, optical spectroscopy, and the like have not been widely implemented in commercial bath analysis systems. Titrimetric and gravimetric techniques are more widely used than chromatographic methods, but these methods require the use of various additional chemistries (titrants, complexants, precipitants) and are difficult to implement in an on-line, real-time configuration
There is a continuing need for the analysis of brightener and leveler concentration in plating baths, that works over a wide range of brightener concentrations, is simple to perform and does not require the addition of other components to the bath for the procedure to be performed.
The present invention overcomes the deficiencies of the conventional analytical methods of cyclic voltammetric stripping (xe2x80x9cCVSxe2x80x9d) and cyclic pulse voltammetric stripping (xe2x80x9cCPVSxe2x80x9d) for the analysis of plating baths containing both leveler and brightener. It has been surprisingly found that the present method provides a simple analytical procedure for determining the quantity of both brightener and leveler in a plating bath, can be performed as a real-time analysis, and can be used with a wide range of brightener concentrations.
The present invention provides a method for determining the quantity of both brightener and leveler in an electroplating bath including the steps of: a) determining the amount of brightener by a method selected from cyclic voltammetric stripping and cyclic pulse voltammetric stripping; b) obtaining a plurality of plating baths where each bath has a known and different quantity of said brightener and leveler, but where the quantity of each in each bath differs from the quantity in the other baths; c) for each bath, providing a counter electrode, a cleaned working electrode and a reference electrode immersed in said bath, and carrying out a predetermined sequence of steps including: 1. cleaning and oxidizing the surface of the working electrode at a fixed potential for a period of time; 2. equilibrating said working electrode to absorb brightener according to a step selected from equilibrating without energy input for a time until the change in energy output with time is minimal or equilibrating for a set time at a fixed potential; 3. plating metal ions on said working electrode with energy input for a time sufficient to measure initial plating energy output; and 4. optionally continuing to plate metal ions for a time sufficient to measure the change in energy output with time; 5. stripping at a potential and for a period of time sufficient to remove the metal ions plated in steps 3 and 4; d) for each bath, correlating the quantity of leveler with the energy output value obtained in step 3 or 4; e) obtaining a plating bath having an unknown quantity of brightener and leveler, placing said electrodes in said bath and performing said predetermined sequence of steps; f) choosing from said correlations in step d), a particular correlation for a bath containing substantially the amount of brightener determined in step a); and g) choosing from the particular correlation in step f), a quantity of leveler which corresponds to said energy outputs recorded for said bath with the unknown quantity of brightener and leveler.
The present invention also provides a method for determining the quantity of both brightener and leveler in an electroplating bath including the steps of: a) obtaining a plurality of plating baths, where each bath has a known and different quantity of said brightener and leveler, but where the quantity of each in each bath differs from the quantity in the other baths; b) sweeping for each of said baths an inert, working electrode at a predetermined rate through a plurality of voltammetric cycles until a condition of steady state is obtained, each of said voltammetric cycles including a metal plating range and a metal stripping range for each of said baths of said plurality of baths, each of said voltammetric cycles comprising a sweeping of a voltage toward one polarity followed by a sweeping of said voltage toward a reverse of said one polarity to complete said cycle; c) measuring the coulombs utilized during said metal stripping range of said cycle for each of said baths of said plurality of baths, whereby a correlation is obtained between the effective quantity of brightener and said coulombs utilized during said metal stripping range; d) obtaining a bath having an unknown quantity of both brightener and leveler; e) sweeping for said unknown bath an inert, working electrode at said predetermined rate through a plurality of voltammetric cycles until a condition of steady state is obtained, each of said voltammetric cycles including a metal plating range and a metal stripping range for said bath having an unknown quantity of brightener, each of said voltammetric cycles comprising a sweeping of a voltage toward one polarity followed by a sweeping of said voltage toward a reverse of said one polarity to complete said cycle; f) measuring the coulombs utilized during said metal stripping range of said cycle for said bath having an unknown quantity of brightener; g) choosing from said correlation a quantity of brightener which corresponds to said coulombs utilized for said bath having an unknown quantity of organic leveling agent; h) for each of said plurality of plating baths in step a), providing a counter electrode, a cleaned working electrode and a reference electrode immersed in said bath, and carrying out a predetermined sequence of steps including: 1. cleaning and oxidizing the surface of the working electrode at a fixed potential for a period of time; 2. equilibrating said working electrode to absorb brightener according to a step selected from equilibrating without energy input for a time until the change in energy output with time is minimal or equilibrating for a set time at a fixed potential; 3. plating metal ions on said working electrode with energy input for a time sufficient to measure initial plating energy output; and 4. optionally continuing to plate metal ions for a time sufficient to measure the change in energy output with time; 5. stripping at a potential and for a period of time sufficient to remove the metal ions plated in steps 3 and 4; i) for each bath, correlating the quantity of leveler with the energy output value obtained in step 3 or 4; j) obtaining a plating bath having an unknown quantity of brightener and leveler, placing said electrodes in said bath and performing said predetermined sequence of steps; k) choosing from said correlations in step i), a particular correlation for a bath containing substantially the amount of brightener determined in step g); and 1) choosing from the particular correlation in step k), a quantity of leveler which corresponds to said energy outputs recorded for said bath with the unknown quantity of brightener and leveler.
The present invention further provides a method for determining the quantity of leveler in an electroplating bath including the steps of: a) obtaining a plurality of plating baths where each bath has a known and different quantity of brightener and leveler, wherein the quantity of leveler in each bath differs from the quantity in the other baths; b) for each bath, providing a counter electrode, a cleaned working electrode and a reference electrode immersed in said bath, and carrying out a predetermined sequence of steps including: 1. cleaning and oxidizing the surface of the working electrode at a fixed potential for a period of time; 2. equilibrating said working electrode to absorb brightener according to a step selected from equilibrating without energy input for a time until the change in energy output with time is minimal or equilibrating for a set time at a fixed potential; 3. plating metal ions on said working electrode with energy input for a time sufficient to measure initial plating energy output; and 4. optionally continuing to plate metal ions for a time sufficient to measure the change in energy output with time; 5. stripping at a potential and for a period of time sufficient to remove the metal ions plated in steps 3 and 4; c) for each bath, correlating the quantity of leveler with the energy output value obtained in step 3 or 4; d) obtaining a plating bath having an unknown quantity of leveler; e) diluting the bath having an unknown quantity of leveler with a fixed concentration of brightener and placing said electrodes in said bath and performing said predetermined sequence of steps; f) choosing from said correlation in step c), a quantity of leveler which corresponds to said energy outputs recorded for said bath with the unknown quantity of leveler.